+ hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization.patch added to -mm tree

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The patch titled
     Subject: hugetlbfs: use i_mmap_rwsem for more pmd sharing synchronization
has been added to the -mm tree.  Its filename is
     hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization.patch

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------------------------------------------------------
From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Subject: hugetlbfs: use i_mmap_rwsem for more pmd sharing synchronization

Patch series "hugetlbfs: use i_mmap_rwsem for better synchronization".

These patches are a follow up to the RFC,
http://lkml.kernel.org/r/20181024045053.1467-1-mike.kravetz@xxxxxxxxxx
Comments made by Naoya were addressed.

There are two primary issues addressed here:

1) For shared pmds, huge PE pointers returned by huge_pte_alloc can
   become invalid via a call to huge_pmd_unshare by another thread.

2) hugetlbfs page faults can race with truncation causing invalid
   global reserve counts and state.

Both issues are addressed by expanding the use of i_mmap_rwsem.

These issues have existed for a long time.  They can be recreated with a
test program that causes page fault/truncation races.  For simple
mappings, this results in a negative HugePages_Rsvd count.  If racing with
mappings that contain shared pmds, we can hit "BUG at
fs/hugetlbfs/inode.c:444!" or Oops!  as the result of an invalid memory
reference.

I broke up the larger RFC into separate patches addressing each issue. 
Hopefully, this is easier to understand/review.


This patch (of 3):

While looking at BUGs associated with invalid huge page map counts, it was
discovered and observed that a huge pte pointer could become 'invalid' and
point to another task's page table.  Consider the following:

A task takes a page fault on a shared hugetlbfs file and calls
huge_pte_alloc to get a ptep.  Suppose the returned ptep points to a
shared pmd.

Now, another task truncates the hugetlbfs file.  As part of truncation, it
unmaps everyone who has the file mapped.  If the range being truncated is
covered by a shared pmd, huge_pmd_unshare will be called.  For all but the
last user of the shared pmd, huge_pmd_unshare will clear the pud pointing
to the pmd.  If the task in the middle of the page fault is not the last
user, the ptep returned by huge_pte_alloc now points to another task's
page table or worse.  This leads to bad things such as incorrect page
map/reference counts or invalid memory references.

To fix, expand the use of i_mmap_rwsem as follows:

- i_mmap_rwsem is held in read mode whenever huge_pmd_share is called. 
  huge_pmd_share is only called via huge_pte_alloc, so callers of
  huge_pte_alloc take i_mmap_rwsem before calling.  In addition, callers
  of huge_pte_alloc continue to hold the semaphore until finished with the
  ptep.

- i_mmap_rwsem is held in write mode whenever huge_pmd_unshare is called.

Link: http://lkml.kernel.org/r/20181203200850.6460-2-mike.kravetz@xxxxxxxxxx
Fixes: 39dde65c9940 ("shared page table for hugetlb page")
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Prakash Sangappa <prakash.sangappa@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/hugetlb.c        |   70 ++++++++++++++++++++++++++++++++----------
 mm/memory-failure.c |   14 +++++++-
 mm/migrate.c        |   13 +++++++
 mm/rmap.c           |    3 +
 mm/userfaultfd.c    |   11 +++++-
 5 files changed, 91 insertions(+), 20 deletions(-)

--- a/mm/hugetlb.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization
+++ a/mm/hugetlb.c
@@ -3239,6 +3239,7 @@ int copy_hugetlb_page_range(struct mm_st
 	int cow;
 	struct hstate *h = hstate_vma(vma);
 	unsigned long sz = huge_page_size(h);
+	struct address_space *mapping = vma->vm_file->f_mapping;
 	unsigned long mmun_start;	/* For mmu_notifiers */
 	unsigned long mmun_end;		/* For mmu_notifiers */
 	int ret = 0;
@@ -3252,11 +3253,23 @@ int copy_hugetlb_page_range(struct mm_st
 
 	for (addr = vma->vm_start; addr < vma->vm_end; addr += sz) {
 		spinlock_t *src_ptl, *dst_ptl;
+
 		src_pte = huge_pte_offset(src, addr, sz);
 		if (!src_pte)
 			continue;
+
+		/*
+		 * i_mmap_rwsem must be held to call huge_pte_alloc.
+		 * Continue to hold until finished  with dst_pte, otherwise
+		 * it could go away if part of a shared pmd.
+		 *
+		 * Technically, i_mmap_rwsem is only needed in the non-cow
+		 * case as cow mappings are not shared.
+		 */
+		i_mmap_lock_read(mapping);
 		dst_pte = huge_pte_alloc(dst, addr, sz);
 		if (!dst_pte) {
+			i_mmap_unlock_read(mapping);
 			ret = -ENOMEM;
 			break;
 		}
@@ -3271,8 +3284,10 @@ int copy_hugetlb_page_range(struct mm_st
 		 * after taking the lock below.
 		 */
 		dst_entry = huge_ptep_get(dst_pte);
-		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry))
+		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry)) {
+			i_mmap_unlock_read(mapping);
 			continue;
+		}
 
 		dst_ptl = huge_pte_lock(h, dst, dst_pte);
 		src_ptl = huge_pte_lockptr(h, src, src_pte);
@@ -3321,6 +3336,8 @@ int copy_hugetlb_page_range(struct mm_st
 		}
 		spin_unlock(src_ptl);
 		spin_unlock(dst_ptl);
+
+		i_mmap_unlock_read(mapping);
 	}
 
 	if (cow)
@@ -3772,14 +3789,18 @@ retry:
 			};
 
 			/*
-			 * hugetlb_fault_mutex must be dropped before
-			 * handling userfault.  Reacquire after handling
-			 * fault to make calling code simpler.
+			 * hugetlb_fault_mutex and i_mmap_rwsem must be
+			 * dropped before handling userfault.  Reacquire
+			 * after handling fault to make calling code simpler.
 			 */
 			hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping,
 							idx, haddr);
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+			i_mmap_unlock_read(mapping);
+
 			ret = handle_userfault(&vmf, VM_UFFD_MISSING);
+
+			i_mmap_lock_read(mapping);
 			mutex_lock(&hugetlb_fault_mutex_table[hash]);
 			goto out;
 		}
@@ -3927,6 +3948,11 @@ vm_fault_t hugetlb_fault(struct mm_struc
 
 	ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
 	if (ptep) {
+		/*
+		 * Since we hold no locks, ptep could be stale.  That is
+		 * OK as we are only making decisions based on content and
+		 * not actually modifying content here.
+		 */
 		entry = huge_ptep_get(ptep);
 		if (unlikely(is_hugetlb_entry_migration(entry))) {
 			migration_entry_wait_huge(vma, mm, ptep);
@@ -3934,20 +3960,31 @@ vm_fault_t hugetlb_fault(struct mm_struc
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
 			return VM_FAULT_HWPOISON_LARGE |
 				VM_FAULT_SET_HINDEX(hstate_index(h));
-	} else {
-		ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
-		if (!ptep)
-			return VM_FAULT_OOM;
 	}
 
+	/*
+	 * Acquire i_mmap_rwsem before calling huge_pte_alloc and hold
+	 * until finished with ptep.  This prevents huge_pmd_unshare from
+	 * being called elsewhere and making the ptep no longer valid.
+	 *
+	 * ptep could have already be assigned via huge_pte_offset.  That
+	 * is OK, as huge_pte_alloc will return the same value unless
+	 * something changed.
+	 */
 	mapping = vma->vm_file->f_mapping;
-	idx = vma_hugecache_offset(h, vma, haddr);
+	i_mmap_lock_read(mapping);
+	ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
+	if (!ptep) {
+		i_mmap_unlock_read(mapping);
+		return VM_FAULT_OOM;
+	}
 
 	/*
 	 * Serialize hugepage allocation and instantiation, so that we don't
 	 * get spurious allocation failures if two CPUs race to instantiate
 	 * the same page in the page cache.
 	 */
+	idx = vma_hugecache_offset(h, vma, haddr);
 	hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping, idx, haddr);
 	mutex_lock(&hugetlb_fault_mutex_table[hash]);
 
@@ -4035,6 +4072,7 @@ out_ptl:
 	}
 out_mutex:
 	mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+	i_mmap_unlock_read(mapping);
 	/*
 	 * Generally it's safe to hold refcount during waiting page lock. But
 	 * here we just wait to defer the next page fault to avoid busy loop and
@@ -4639,10 +4677,12 @@ void adjust_range_if_pmd_sharing_possibl
  * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
  * and returns the corresponding pte. While this is not necessary for the
  * !shared pmd case because we can allocate the pmd later as well, it makes the
- * code much cleaner. pmd allocation is essential for the shared case because
- * pud has to be populated inside the same i_mmap_rwsem section - otherwise
- * racing tasks could either miss the sharing (see huge_pte_offset) or select a
- * bad pmd for sharing.
+ * code much cleaner.
+ *
+ * This routine must be called with i_mmap_rwsem held in at least read mode.
+ * For hugetlbfs, this prevents removal of any page table entries associated
+ * with the address space.  This is important as we are setting up sharing
+ * based on existing page table entries (mappings).
  */
 pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
 {
@@ -4659,7 +4699,6 @@ pte_t *huge_pmd_share(struct mm_struct *
 	if (!vma_shareable(vma, addr))
 		return (pte_t *)pmd_alloc(mm, pud, addr);
 
-	i_mmap_lock_write(mapping);
 	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
 		if (svma == vma)
 			continue;
@@ -4689,7 +4728,6 @@ pte_t *huge_pmd_share(struct mm_struct *
 	spin_unlock(ptl);
 out:
 	pte = (pte_t *)pmd_alloc(mm, pud, addr);
-	i_mmap_unlock_write(mapping);
 	return pte;
 }
 
@@ -4700,7 +4738,7 @@ out:
  * indicated by page_count > 1, unmap is achieved by clearing pud and
  * decrementing the ref count. If count == 1, the pte page is not shared.
  *
- * called with page table lock held.
+ * Called with page table lock held and i_mmap_rwsem held in write mode.
  *
  * returns: 1 successfully unmapped a shared pte page
  *	    0 the underlying pte page is not shared, or it is the last user
--- a/mm/memory-failure.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization
+++ a/mm/memory-failure.c
@@ -1028,7 +1028,19 @@ static bool hwpoison_user_mappings(struc
 	if (kill)
 		collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
 
-	unmap_success = try_to_unmap(hpage, ttu);
+	if (!PageHuge(hpage)) {
+		unmap_success = try_to_unmap(hpage, ttu);
+	} else {
+		/*
+		 * For hugetlb pages, try_to_unmap could potentially call
+		 * huge_pmd_unshare.  Because of this, take semaphore in
+		 * write mode here and set TTU_RMAP_LOCKED to indicate we
+		 * have taken the lock at this higer level.
+		 */
+		i_mmap_lock_write(mapping);
+		unmap_success = try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
+		i_mmap_unlock_write(mapping);
+	}
 	if (!unmap_success)
 		pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
 		       pfn, page_mapcount(hpage));
--- a/mm/migrate.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization
+++ a/mm/migrate.c
@@ -1297,8 +1297,19 @@ static int unmap_and_move_huge_page(new_
 		goto put_anon;
 
 	if (page_mapped(hpage)) {
+		struct address_space *mapping = page_mapping(hpage);
+
+		/*
+		 * try_to_unmap could potentially call huge_pmd_unshare.
+		 * Because of this, take semaphore in write mode here and
+		 * set TTU_RMAP_LOCKED to let lower levels know we have
+		 * taken the lock.
+		 */
+		i_mmap_lock_write(mapping);
 		try_to_unmap(hpage,
-			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS|
+			TTU_RMAP_LOCKED);
+		i_mmap_unlock_write(mapping);
 		page_was_mapped = 1;
 	}
 
--- a/mm/rmap.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization
+++ a/mm/rmap.c
@@ -1374,6 +1374,9 @@ static bool try_to_unmap_one(struct page
 		/*
 		 * If sharing is possible, start and end will be adjusted
 		 * accordingly.
+		 *
+		 * If called for a huge page, caller must hold i_mmap_rwsem
+		 * in write mode as it is possible to call huge_pmd_unshare.
 		 */
 		adjust_range_if_pmd_sharing_possible(vma, &start, &end);
 	}
--- a/mm/userfaultfd.c~hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization
+++ a/mm/userfaultfd.c
@@ -267,10 +267,14 @@ retry:
 		VM_BUG_ON(dst_addr & ~huge_page_mask(h));
 
 		/*
-		 * Serialize via hugetlb_fault_mutex
+		 * Serialize via i_mmap_rwsem and hugetlb_fault_mutex.
+		 * i_mmap_rwsem ensures the dst_pte remains valid even
+		 * in the case of shared pmds.  fault mutex prevents
+		 * races with other faulting threads.
 		 */
-		idx = linear_page_index(dst_vma, dst_addr);
 		mapping = dst_vma->vm_file->f_mapping;
+		i_mmap_lock_read(mapping);
+		idx = linear_page_index(dst_vma, dst_addr);
 		hash = hugetlb_fault_mutex_hash(h, dst_mm, dst_vma, mapping,
 								idx, dst_addr);
 		mutex_lock(&hugetlb_fault_mutex_table[hash]);
@@ -279,6 +283,7 @@ retry:
 		dst_pte = huge_pte_alloc(dst_mm, dst_addr, huge_page_size(h));
 		if (!dst_pte) {
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+			i_mmap_unlock_read(mapping);
 			goto out_unlock;
 		}
 
@@ -286,6 +291,7 @@ retry:
 		dst_pteval = huge_ptep_get(dst_pte);
 		if (!huge_pte_none(dst_pteval)) {
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+			i_mmap_unlock_read(mapping);
 			goto out_unlock;
 		}
 
@@ -293,6 +299,7 @@ retry:
 						dst_addr, src_addr, &page);
 
 		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+		i_mmap_unlock_read(mapping);
 		vm_alloc_shared = vm_shared;
 
 		cond_resched();
_

Patches currently in -mm which might be from mike.kravetz@xxxxxxxxxx are

hugetlbfs-use-i_mmap_rwsem-for-more-pmd-sharing-synchronization.patch
hugetlbfs-use-i_mmap_rwsem-to-fix-page-fault-truncate-race.patch
hugetlbfs-remove-unnecessary-code-after-i_mmap_rwsem-synchronization.patch




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